1. Field of the Invention
The present invention is directed generally to power distribution units.
2. Description of the Related Art
Single-phase electrical devices may be electrically coupled to a three-phase system, either by connecting between a phase conductor and the system neutral (line-to-neutral) or by connecting across two phase conductors in a phase-to-phase connection. In North America, lighting and small appliance outlets are typically connected line-to-neutral to give the 120 V distribution voltage (115V utilization voltage), and high-power loads such as cooking equipment, space heating, water heaters, or air conditioning can be connected across two phases in a phase-to-phase connection to provide approximately 208 V. The use of 208 V is widespread enough that in North America 208 V single-phase equipment can be relatively easy to find. The 208 V connection is preferred for high-power appliances because it can provide almost double the amount of power for the same current and wire size. When power demands increase, it is possible to reconfigure existing wiring in a building in the United States from a standard line-to-neutral 120 V single phase power to phase-to-phase 208 V single phase power.
Conventional power distribution units generally have a plurality of electrical outlets also known as electrical receptacles that couple to electrical power to furnish electrical power to devices electrically coupled to the receptacles. For 208 V electrical distribution, the electrical outlets typical have three openings to receive three prongs of the electrical plugs. A first prong of each of the plugs is used to electrically couple the device to an electrical ground. A second prong of each of the plugs is used to electrically couple the device to a first phase (such as an x-phase) related part of an electrical power source. A third prong of each of the plugs is to electrically couple the device to a second phase (such as a y-phase) related part of the electrical power source. The power distribution units have two separate circuit portions that electrically couple first phase and second phase related portions, respectively, of the electrical power source to each of the plurality of electrical outlets.
To address concerns including those related to safety, a conventional power distribution unit may use one or two circuit breakers each of a single pole type that are each electrically coupled to a different one of the first phase and second phase circuit portions of the power distribution unit. In a first case of conventional use, only one single pole circuit breaker is used for a power distribution unit. For this first case, the one single pole circuit breaker is electrically coupled to one of the circuit portions associated with one of the two phases of an electrical power source. For example, in this first case, the x-phase circuit portion of the power distribution unit may be coupled to a sole single pole circuit breaker whereas the y-phase circuit portion of the power distribution unit may not be coupled to any circuit breaker at all.
In a second case of conventional use, two single pole circuit breakers, a first single pole circuit breaker and a second single pole circuit breaker, are used for the power distribution unit. For this second case, the first single pole circuit breaker can be coupled to the x-phase circuit portion of the power distribution unit. Furthermore, for this second case, the second single pole circuit breaker can be coupled to the y-phase circuit portion of the power distribution unit. For example, the first single pole breaker could be coupled to the x-phase circuit portion of the power distribution unit.
In the first case, where a power distribution unit uses only one single pole breaker, a dangerous situation could occur. As an example, the sole single pole circuit breaker could be electrically coupled to the x-phase circuit portion of the power distribution unit whereas in this case the y-phase circuit portion of the power distribution unit would be not electrically coupled to a circuit breaker. If a condition developed that caused the sole single pole circuit breaker to trip open, then the x-phase circuit portion of the power distribution unit would not be electrically coupled to the x-phase portion of electrical power.
In this example of the first case, if a user made electrical contact with the x-phase circuit portion of the power distribution unit and also made electrical contact with a relative electrical ground, the user would receive no electrical current from the x-phase portion of the electrical power source. On the other hand, in this first case, the y-phase circuit portion of the electrical power distribution unit is not electrically coupled to a circuit breaker and consequently remains electrically coupled to the y-phase portion of the electrical power source.
In this example of the first case, if a user made electrical contact with the y-phase circuit portion of the power distribution unit (in particular making electrical contact with the associated y-phase conductor of one of the electrical outlets) and also made electrical contact with a relative electrical ground, the user would receive undesired electrical current from the y-phase portion of the electrical power source. This could be quite a surprise to the user, since the user may assume that since the sole single pole circuit breaker was tripped open that both the x-phase circuit portion of the power distribution unit was not electrically coupled to the x-phase portion of the electrical power source and the y-phase circuit portion of the power distribution unit was not electrically coupled to the y-phase portion of the electrical power source.
In the second case, where a power distribution unit uses one single pole circuit breaker for the x-phase circuit portion of the power distribution unit and uses another single pole circuit breaker for the y-phase circuit portion of the power distribution unit, a similarly dangerous situation could occur. As an example, a first single pole circuit breaker could be electrically coupled to the x-phase circuit portion of the power distribution unit and a second single pole circuit breaker could be electrically coupled to the y-phase circuit portion of the power distribution unit.
An electrical condition, such as an overcurrent condition, could develop that causes the first single pole circuit breaker to trip open, so that the x-phase circuit portion of the power distribution unit would not be electrically coupled to the x-phase portion of the electrical power source. However, for this same electrical condition, the second single pole circuit breaker may not be tripped open, so that the y-phase circuit portion of the power distribution unit would still be electrically coupled to the y-phase portion of the electrical power source.
In this example of the second case, if a user made electrical contact with the x-phase circuit portion of the power distribution unit and also made electrical contact with a relative electrical ground, the user would receive no electrical current from the x-phase portion of the electrical power source. On the other hand, the y-phase circuit portion of the electrical power distribution unit remains electrically coupled to the y-phase portion of the electrical power source.
In this example of the second case, if a user made electrical contact with the y-phase circuit portion (in particular making electrical contact with the associated y-phase conductor of one of the electrical outlets) of the power distribution unit and also made electrical contact with a relative electrical ground, the user would receive undesired electrical current from the y-phase portion of the electrical power source. This could be quite a surprise to the user, since the user may assume that both of the two circuit breakers found in the power distribution unit have tripped open or the user may assume that since one of the two circuit breakers has tripped open that the power distribution unit is somehow in any case not electrically coupled to a portion of the electrical power source.